One of the well-known mass-analyzing methods using a triple quadrupole mass spectrometer (TQMS) or an ion trap time-of-flight mass spectrometer (IT-TOFMS) is an MS/MS analysis (or tandem analysis). In a general MS/MS analysis, an ion having a specific mass-to-charge ratio (m/z) is first selected as a precursor ion from a substance to be analyzed. Next, the precursor ion thus selected is dissociated into product ions (also referred to as fragment ions) having a small mass-to-charge ratio by a CID (Collision Induced Dissociation) process. The forms of dissociation depend on the structure of an original compound. Accordingly, the composition and chemical structure of a sample molecule can be estimated by conducting a mass spectrometry of various product ions generated by dissociation and creating an MS/MS (=MS2) spectrum.
In recent years, the molecular weight of a substance to be analyzed by such a mass spectrometer tends to become larger and the structure (composition) tends to be more complicated. Accordingly, in some cases depending on a certain property of a substance, the ion of the substance cannot be dissociated to a sufficiently small mass-to-charge ratio with a single-stage dissociating operation. In such cases, the dissociating operations may sometimes be repeated multiple times (n−1 times make an MSn analysis) and finally produced product ions are mass-analyzed (see e.g., JP H10-142196 A and JP 2001-249114 A). Mass analysis through a single-stage dissociating operation as described above is an MS/MS analysis, or an MS2 analysis. In this description, mass spectrometer capable of MSn analysis where n is two or larger is referred to as an MSn mass spectrometer.
In the cases of identifying an unknown substance and of estimating the chemical structure from data (MSn spectral data) obtained by MSn analysis, a database is usually searched based on a fragmentation pattern in an MS spectrum (spectrum obtained by mass spectrometric analysis without ion dissociation) and an MSn spectrum. In the database (sometimes also referred to as a library) used in such cases, various information on various known compounds, that is, compound names, molecular weights, composition formulae, structural formulae, MSn spectral data, partial structures corresponding to various product ion peaks appearing in MSn spectra and the like are collected. The databases may be, for example, standard databases, such as NIST, Wiley and Drug, or databases prepared by analytical instrument manufacturers. It may also be constructed by users.
In the database search, among enormous amount of compounds collected in the database, compounds having MSn spectra that can be considered to match with those having the peak patterns of MSn spectra derived from a substance to be identified are searched. The retrieved compounds are listed as options. Scores are assigned to respective options according to a degree of matching. The options are normally displayed in the descending order of scores.
FIG. 6A and FIG. 6B are examples of display screens showing search results of database search as described above. In the examples, the upper part of a display screen 60 is a search result list display area 61, and a spectrum display area 62 is provided below. In the search result list display area 61, the registration numbers of MSn spectra hit in the database search, and the names of compounds from which the spectra are obtained are displayed in a list. In the spectrum display area 62, the MSn spectrum obtained by analysis of a target sample, and a reference MSn spectrum are displayed in a vertical comparison. Here, the MSn spectrum as the comparison reference is one selected by a user from among the MSn spectra listed in the search result list display area 61.
The mass-to-charge ratio ranges, that is, the minimum values and the maximum values on the abscissa of the spectra, of the MSn spectrum of the target sample (hereinafter, referred to as a “target spectrum”) and of the MSn spectrum as the comparison reference (hereinafter, referred to as a “comparison reference spectrum”) do not match, in general, with each other. For example, in the case of FIG. 6A and FIG. 6B, the mass-to-charge ratio range of the target spectrum 63 is m/z: 0 to 1200 and that of the comparison reference spectrum 64 is m/z: 200 to 800. If these spectra are vertically arranged on the display screen 60 with the same width as shown in FIG. 6A, the abscissa scales of the two spectra do not match. Accordingly, even if the two spectra include peaks having the same m/z (common peaks), it is difficult for a user to grasp the fact at a glance.
To address this, conventional mass spectrometry data processing apparatuses had a function of matching the abscissa scales of the two spectra in such cases. For example, in the aforementioned example, if the user selects “Match on Maximum Width” from among radio buttons provided in a spectrum matching method selection field 65 on the right side of the spectrum display area 62, the target spectrum 63 and the comparison reference spectrum 64 are rearranged so that the abscissa scales are matched with each other as shown in FIG. 6B. Specifically, the mass-to-charge ratio ranges on the abscissa of the two spectra are compared, and their minimum value (m/z: 0 in the example) and maximum value (m/z: 1200 in the example) are detected, which are determined as a target minimum value and a target maximum value, respectively. Subsequently, in conformity with the target minimum value and the target maximum value, the abscissa axis of the target spectrum 63 and/or the abscissa axis of the comparison reference spectrum 64 are extended (in the example, the comparison reference spectrum 64 is extended from a state where the minimum value is m/z: 200 and the maximum value is m/z: 800 to a state where the minimum value is m/z: 0 and the maximum value is m/z: 1200). The two spectra thus adjusted are displayed on the display screen 60 with the same width as shown in FIG. 6B, where the scales of the two spectra match on the abscissa with each other. Consequently, if the target spectrum 63 and the comparison reference spectrum 64 include peaks having the same m/z (common peaks), the common peaks (the peak at m/z: 500 and the peak at m/z: 650 in the example of FIG. 6B) are displayed aligned on a vertical line on the display screen, thereby allowing the user to easily grasp this alignment.